Thyroid ultrasound: Thyroid nodule with eggshell calcification and posterior signal dropout

Posterior signal dropout due to a linear calcification (eggshell calcification). All of the ultrasound waves are reflected back to the probe upon encountering a calcification, which appears as a bright, curved line (arrow). The calcification inhibits the sound waves from traveling through the remaining tissue and the image posterior to the lesion is consequently darker (dashed arrows).